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Technology Paper
L1 RTK System with Fixed Ambiguity:
What SBAS Ranging Brings
L1 RTK System with Fixed Ambiguity:
What SBAS Ranging Brings
Alexey Boriskin, Dmitry Kozlov, Gleb Zyryanov
Magellan, Russia
BIOGRAPHY
Alexey Boriskin has been working in Magellan since The data used for validation were collected with Magellan
2005 as a Software Engineer. He received his MS EE ProMark3 and DG14 receivers supporting L1 GPS+SBAS
degree from Moscow Aviation Institute. He is also post RTK.
graduated student in Moscow Aviation Institute.
INTRODUCTION
Dmitry Kozlov has been working in Magellan since 1993
as a Senior Scientist. Since 2002 he is Algorithm Group
SBAS ranging signal is the same as GPS signal [1]. This
Manager. He received his MS EE degree from Moscow
means that corresponded pseudo-range and carrier phase
Aviation Institute and his PhD in Signal Processing
measurements must be equivalent to GPS L1 CA
Theory from the Institute of Automatics, Moscow.
measurements. The only principal difference between
SBAS and GPS is different navigation data; particularly
Gleb Zyryanov has been working in Magellan since 2000
SBAS orbits and clock corrections are computed
as a Software Engineer. Since 2006 he is Senior Software
differently from GPS.
Engineer. He received his MS in Mathematics and
Mechanics from Moscow State University.
Currently there are three operating SBAS constellations:
ABSTRACT • WAAS, which includes 2 Satellites covering
North and South America and parts of the Pacific
Given paper deals with centimeter level L1 RTK systems. ocean.
L1 and L1&L2 RTK provide the same centimeter level • EGNOS, which includes 3 Satellites covering
accuracy for short baselines. However, unlike expensive Europe and Africa and some nearby countries.
L1/L2 RTK, with cheaper L1 RTK one cannot expect too • MSAS, which includes 2 Satellites covering
fast (seconds) On-The-Fly (OTF) ambiguity resolution, Japan, China and Australia.
which delivers cm accuracy.
In some areas one can see and track 2+ SBAS satellites,
Known disadvantages of L1 RTK system (compared to say in California (US) 4 WAAS could be seen at the same
L1&L2) are baseline length restriction (typically 10 km) time until July 2007.
and noticeable performance degradation under shaded
sky. Augmented GPS constellation can mitigate these From the point of signal quality and maturity of orbital
disadvantages. information, WAAS and MSAS Satellites are good. On
the contrary the EGNOS signal is not yet stable, and the
It is known that SBAS Satellites provide not only accuracy of the provided orbital data is currently poor.
long/fast/ionosphere corrections. They are also a source of That is why usage of SBAS (especially EGNOS)
GPS-like signal which pseudo range and carrier phase measurements in position computation is a challenge.
measurements can be potentially used in positioning
together with GPS measurements. Given paper proves When speaking about Fixed RTK, one is usually
that SBAS measurements can make a good job to interested in time needed to fix carrier ambiguity and
augment GPS L1 RTK. We give a lot of real life collected achieve cm level solution insuring at the same time preset
statistic (with WAAS, EGNOS and in less degree MSAS) reliability. New L1 RTK solution from Magellan allows
which demonstrate dramatic performance enhancement of using SBAS measurements in RTK process, thus making
L1 RTK thanks to using SBAS. it a true GNSS technology. SBAS gives extra GPS-like
measurements, which improve Satellite geometry and 5. There are also some receiver related issues which
allow achieving cm level accuracy faster compared to can lead to some SBAS measurement biases
GPS only case. between receivers of different
types/manufactures. This is simply the effect of
The GPS+SBAS RTK technique is similar to immaturity of SBAS ranging nowadays.
GPS+GLONAS L1 RTK technique, which was also 6. The most of modern GPS+SBAS receivers are
invented by Magellan (formerly Ashtech) [2], [3]. From not SBAS-all-in-view just because the primary
the point of L1 RTK performance, two extra SBAS function of SBAS is to provide corrections rather
Satellites do the same job as three extra GLONASS than measurements and there is no any need to
Satellites. With the currently incomplete GLONASS have more than 2 SBAS tracking channels.
constellation, the ‘power’ of L1 GPS+SBAS RTK and L1
GPS+GLONASS RTK is approximately the same. Fortunately many existing SBAS ranging disadvantages
are mitigated when SBAS measurements are used in
The paper is organized as follows. differential processing. At the same time, some of the
negative effects still exist, and when processing SBAS
First we describe some specific details when processing measurements one must take care. The new GPS+SBAS
SBAS ranging data along with GPS data in RTK engine. RTK processing technique from Magellan not only uses
SBAS ranging and carrier data, but also takes great care
Then we provide apple-to-apple comparison statistic that a possible SBAS failure does not spoil RTK behavior.
showing the improvement in L1 RTK performance thanks
to acquiring SBAS ranging information. Instead of ‘mechanical’ usage of SBAS ranging in the
RTK processing, Magellan has incorporated the following
After this we overview transporting formats which allow 3 principal innovations:
implementing GPS+SBAS RTK process between base
and rover receivers. • Adaptive SBAS usage
• SBAS data calibration
Finally we present open sky short baseline RTK statistic • SBAS tracking synchronization
we got with different combinations of SBAS enabled
base/rover RTK receivers from Magellan. In many cases (especially with EGNOS), SBAS data can
be bad and under no circumstances must be taken into the
ALGORITHM RTK processing engine. Adaptive SBAS usage means
detecting wrong SBAS measurements and/or orbit and
From very first glance SBAS ranging data (pseudo range stopping their usage. One of the examples is poor
and carrier phase measurements) appear to be very similar ephemeris information. In this case transmitted URA
to GPS ranging data. They follow the very same (User Range Accuracy) is not always adequate because
observation model [4] and therefore can be absorbed into SBAS with very bad URA can be often effectively used in
GPS positioning process as extra GPS Satellites. RTK process, because orbital and/or clock errors can be
acceptable for RTK positioning, while cannot be
However, when trying to acquire SBAS in positioning acceptable for stand alone positioning.
process, one realizes that is it not exactly so. Careful
analysis of SBAS data from point of their usage in SBAS measurements (especially when base and rover
position has shown that the following issues must be data are provided by different receiver types) can have
taken into account. biases which must be accounted for. A special robust
procedure estimates the possible SBAS biases in real time
1. SBAS navigation information is not always and compensates for them in the RTK processing.
accurate. This is clearly seen with EGNOS,
which often provide low quality ephemeris and Usually a receiver is equipped with only 2 channels to
no acceptable clock corrections. track SBAS (e.g., this is the case of DGRTK and
2. SBAS signal is not always stable (again mainly it ProMark3), i.e. it is not an all-in-view SBAS receiver. In
concerns EGNOS). some cases 2+ SBAS satellites can be seen, so it is
3. SBAS constellations was changed many times desirable to track in the rover those SBAS satellites for
(e.g. re-shaping WAAS constellation in 2006 and which the base transmits data. Such an algorithm has been
2007) and is still not fixed at least for EGNOS. implemented, which allows insuring matched SBAS
4. Short term SBAS clock stability is poorer than tracking on base and rover.
that in GPS, which does not allow to extrapolate
SBAS data effectively in time. PERFORMANCE
When demonstrating performance, we will focus on 15 data sets (each at least 24 hours long) for open-sky
statistical figures rather than on presenting particular test baselines from a few tens of meters to 7 km were used.
results. All the data we used for performance evaluation One or two common SBAS satellites were available to
were collected with static receivers. However, RTK was both base and rover. The most of the data were collected
running w/o static assumption (i.e. in kinematics mode). in Europe (EGNOS) and US (WAAS), the last data set
All performance was evaluated with default settings corresponds to China (MSAS).
which were the very same for each processed data set.
The diagram below shows availability for each data set
One very important note must be made. When collecting with and w/o SBAS usage. In all the cases, preset
statistics we used the RTK auto-reset methodology. We reliability was met.
always used fixed-length intervals between RTK resets
regardless its current status. Some vendors provide similar
auto-reset statistics using the float-length intervals, when Availability with and w/o SBAS
RTK reset occurs depending on the current RTK status
(e.g. few seconds after fix). This float-length interval
GPS only
100 GPS+SBAS
approach usually gives a more optimistic statistic
compared to fixed-length interval statistic. Moreover, the
fixed in 300 sec, %
90
fixed-length interval statistic allows comparing in the
same way two different algorithms. That is why we use 80
the fixed-length interval statistic in all cases.
70
Given section demonstrates performance estimated with 60
PC version of GPS+SBAS RTK. Given PC version is
100% adequate to what is running in a receiver. At the 50
same time, section INTEROPERABILITY gives pure real data set
life real time statistic.
To demonstrate Fixed RTK performance we used the Figure 1. Improvement availability thanks to SBAS in
following methodology. RTK rover was reset each 300 open sky conditions
seconds and standard Time To First Fix (TTFF)
performance was evaluated. In given paper we use the One can see that availability of fixed solution at 300 sec
following particular figures of TTFF: interval is about 15% higher due to the addition of SBAS
measurements into RTK process.
Availability == the percentage of fixed trials
over all the trials B. BLOCKED SKY OTF RTK INITIALIZATION
Reliability == the percentage of correctly fixed
trials over all the fixed trials 3 data sets (each at least 24 hours long) for blocked sky
x% point of TTFF == the time within which x% baselines were used. All the data were collected with
of trials were fixed (e.g. x=50,90,99) ProMark3 receivers in California, US, where 4 WAAS
were seen (since July 2007 2 WAAS were disabled) and 3
Each baseline was evaluated with and w/o using SBAS to of them at a good elevation. At given location even with
see apple-to-apple performance. All the data were shaded environment, at least one (and often two) common
collected with 1 second interval. Preset reliability of fixed SBAS satellites were available for each baseline.
solution was set to 99%. Thanks to large data volume for
each particular data set, all our estimates are statistically Used baseline lengths were 1 km, 3.6 km (both partly
sufficient. shaded) and 2 meters (most shaded). The diagram below
shows the availability for each data set.
The RTK performance benefit thanks to using SBAS
ranging information is demonstrated below for 3 most
important cases:
A. Open sky OTF RTK initialization
B. Partly blocked sky OTF RTK initialization
C. RTK initialization with geometry constrains
A. OPEN SKY OTF RTK INITIALIZATION
• ProMark3 RTK receiver when initializing on so
Availability with and w/o SBAS called kinematics bar
• DG RTK when performing Heading
100 GPS only determination
90 GPS+SBAS It is clear that additional constrain brings more
fixed in 300 sec, %
80 information which makes ambiguity fix faster and more
70
60
reliable. Here we show that for such application, the
50 availability of SBAS in RTK process improves TTFF
40 noticeably.
30
20 The diagram below shows 90% point of TTFF for 4 data
10 sets corresponding to baselines of 7, 1, 9, and 20 meters
0
collected with DG RTK receivers in Europe (EGNOS)
data set and US (WAAS). RTK was running in so called RTK
Arrow mode which used the fact that:
Figure 2. Improvement availability thanks to SBAS in
shaded sky conditions • Baseline length is known with sub-cm accuracy
• Baseline elevation does not exceed +/-15 degrees
One can see that for partly shaded baselines SBAS makes
excellent job. With heavy shading the value of SBAS is
very difficult to overestimate. GPS only
TTFF with and w/o SBAS
GPS+SBAS
It should be noted that reliability was not met for most
100
shaded (3rd) baseline.
Time To Fix in 90%,
90
80
The primary Land Survey job is surveying points, i.e. 70
sec 60
processing static observations. It this case RTK can be 50
commanded to work in static mode. Usually for short 40
open sky baselines TTFF performance is quite similar 30
20
when processing data in kinematics and static modes. 10
However with problem data, static assumption can 0
increase performance noticeably. The table below shows data set
how TTFF can be improved when processing the most
shaded 3rd baseline with static assumption. TTFF figures
are given in form GPSonly/GPS+SBAS. Figure 3. Improvement TTFF thanks to SBAS for
RTK on short baseline with known length
Table 1. Combined effect of SBAS usage and static
processing option In all the cases experienced reliability was higher than
Processing Availability, Reliability, TTFF, 50%, 99.9%. One can see again the improvement thanks to
mode Percent Percent seconds SBAS.
Kinematics 8.4 / 42.6 95.4 / 96.4 >300 / >300
Static 13.7 / 56.3 97.3 / 99.7 >300 / 267
TRANSPORTING
One can see that using static assumption in couple with
adding SBAS, has finally allowed to increase availability Obviously, to enable GPS+SBAS RTK processing, a base
and met preset reliability 99%. station must send SBAS data. With standardized
protocols, this is possible when using RTCM-3 format
where a room for SBAS data is reserved [5]. Magellan
C. RTK INITIALIZATION WITH GEOMETRY ProMark3 base/rover RTK receivers support this protocol
CONSTRAINS and can work effectively in GPS+SBAS RTK mode
between each other. At the same time ProMark3 RTK
There are RTK applications when some geometric rover can work against any other RTCM-3 enabled base.
constrains can be used to speed up integer ambiguity However, up to this date we do not know about
initialization. The most known example is initialization on commercial base receivers (e.g. in NTRIP Networks)
baseline with known length. Such an initialization can be which generate SBAS ranging data. That is why
used optionally in: ProMark3 RTK rover shows the best performance against
ProMark3 RTK base which sends SBAS.
or ProMark3) always performed synchronous (with base)
DG RTK rover supports RTCM-3 protocol and can SBAS tracking.
effectively work with ProMark3 RTK base in GPS+SBAS
L1 RTK mode. At the same time, DG14 RTK base The same (as described above) RTK auto-reset
supports RTCM-2 only, which has no room for sending methodology was used to derive TTFF performance. The
SBAS ranging data [6]. So it is not formally possible to diagram below gives the summary TTFF statistic.
broadcast SBAS corrections from a DG14 Base to a
DG14 rover. However, DG RTK base can send SBAS
ranging data in proprietary format. This proprietary TTFF, percent points
format can be decoded by both DG RTK rover and 50%
ProMark3 RTK rover. 300
90%
So ProMark3 RTK and DG RTK are 100% compatible
from point of transporting used for transmission and 200
99%
second
reception of GPS+SBAS raw data. The section below
proves this compatibility.
100
INTEROPERABILITY
0
GPS+SBAS RTK algorithm has been implemented into
data set
latest 2 Magellan products: DG14 OEM board and
ProMark3 handheld Surveyor. While RTK source code is
exactly the same, all the stuff related with deriving raw Figure 4. TTFF for different short baseline tests
SBAS measurements, generating and decoding RTCM
corrections are formally different for these receivers. That One can see that all the ProMark3/DGRTK combinations
is why, compatibility between two formally different are compatible and provide excellent short baseline
SBAS enabled RTK receivers must be checked. GPS+SBAS L1 RTK performance.
As stated above, we do not know commercial RTK bases
which send SBAS ranging data. So ProMark3 and DG CONCLUSIONS
RTK rovers cannot take advantage of GPS+SBAS RTK
processing working with 3rd party RTK bases. At the We have demonstrated statistically that adding SBAS
same time they can effectively work with each other. pseudo range and carrier phase measurement to L1 GPS
RTK improves TTFF performance in very noticeable
8 open sky short baseline (from meters to tens meters) degree. This improvement is just a result of up to 2 extra
RTK tests were performed in Nantes (France), Santa GPS-like L1 measurements into RTK process.
Clara (US) and Moscow (Russia). These tests included all
possible configurations, i.e. In [7], author claimed that L1 real time solution can be
very welcome for low/medium end RTK market as a
• ProMark3-> ProMark3 reasonable competitor of expensive professional L1/L2
• DGRTK-> ProMark3 systems. On short open sky baselines, any extra Satellite
can make L1 RTK initialization noticeably faster. SBAS
• DGRTK-> DGRTK
is the system, which deliver such extra Satellites.
• ProMark3-> DGRTK
SBAS Satellites make revolution job in shaded areas
Receiver operated with all default settings.
where L1 GPS RTK is usually impotent to provide cm
level accuracy. Only augmentation by other GNSS can
Each data set includes more than 24 hours of RTK data.
make L1 RTK workable in difficult environmental
Since receivers can track no more than 2 SBAS
conditions. Earlier it had been proven with
simultaneously while in some cases (e.g. in Santa Clara) 4
GPS+GLONASS, now it has been proven with
SBAS can be potentially seen (test were performed before
GPS+SBAS.
July 2007), we made all these tests under control of
special script with forced base receiver (DGRTK or
Geo-stationary Satellites are entering our life through
ProMark3) to switch from one 2-SBAS combination to
more and more different GNSS systems. Being primary
another each 2 hours. This allowed us to validate
designed as a provider of corrections and other GNSS
interoperability and performance for any SBAS
(and not GNSS) augmentation data, these geo-stationary
configurations. Please note once more that rover (DGRTK
space vehicles insure ‘standard’ navigation ranging signal
which pseudo range and carrier phase can be measured.
These measurements appear to be usable in GNSS
positioning including even such a super-accurate mode as
RTK.
New L1 products from Magellan use SBAS ranging in
RTK process making L1 Fixed RTK productivity much
higher compared to GPS only case.
ACKNOWLEDGMENTS
Authors would like to thank Magellan System Test group
for their careful testing and validation efforts with release
DG RTK and ProMark3 RTK. Our personal thanks for
valuable help with data collection and performance tests
are to Yves Le Pallec, Jean-Charles Torres, Joe Sass,
Eugeny Sunitsky, Phil Stevenson (all Magellan) and Bill
Cottrel (Cottrel Navigation Services).
REFERENCES
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[2] “Centimeter Level Real-Time Kinematic Positioning
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Tkachenko, Navigation: Journal of the Institute of
Navigation, vol.45, No.2, Summer 1998, pp. 137-147.
[3] D. Kozlov, A. Povaliaev, L. Rapoport, S. Sila-
Novitsky, V. Yefriemov, “Relative Position Measuring
Techniques Using Both GPS and GLONASS Carrier
Phase Measurements”, US Patent No. 5,914,685, Jun. 22,
1999.
[4] Leick. A., GPS Satellite Surveying, John Wiley &
Sons, Inc., 1995, 2nd ed.
[5] RTCM STANDARD FOR DIFFERENTIAL GNSS
SERVICES – VERSION 3, RTCM SPECIAL
COMMITTEE NO. 104, AUGUST 11, 2006
[6] RTCM STANDARD FOR DIFFERENTIAL GNSS
SERVICE - VERSION 2.3, RTCM SPECIAL
COMMITTEE NO. 104, AUGUST 20, 2001
[7] “Big Mo, Huge Mo, and No Mo”, Eric Gakstatter,
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